Push-button dialling has now become the norm in telephone instrument design, and the present invention seeks to provide an improved security device serving to prevent access to the push-buttons in order to prevent or control unauthorised usage of the telephone. Push-button locks are known from the prior art, for example, U.S. Pat. Specification No. 4 451,713 describes a lock for two adjacent push-buttons in a push-button array. U.S. Pat. Specification No. 4,398,067 describes a lock for inactivating a single push-button. German Patent Specifications Nos. 3112715A and 3205626A describe a small housing adapted to cover the array of push-buttons which can be locked in engagement onto the telephone instrument. Apertures may be provided for access to one or more push-buttons to allow limited dialling of certain numbers. All of these arrangements require that the telephone instrument itself has to be specially modified to enable the particular housing or locking device to engage with the telephone instrument by drilling holes or by modifying individual push-buttons. For example, in European Patent Specification No. 0,074,617A, a push-button housing is described including a lock which engages one of the push-buttons which must be specially modified by cutting grooves in the sides thereof. Many of these prior art devices, some of which rely on frictional engagement, are not particularly secure, and modification of the telephone instrument is undesirable, and may not be permitted if the telephone is leased from a telephone company.